I have been searching for some type of high protein vegan ready to eat snacks (I'm thinking like crisps, or maybe just flavoured crispy lentils or something) which can be purchased in bulk and have a reasonable expiration date (i.e. at least over a week).

The good old, shelled or unshelled, salted or spiced peanut is hard to beat for price to protein ratio and wide availability. Of course, it is high in fat and sodium too. Watch out for (nowadays rare) brands using gelatin as a coating aid...

Dry roasted soybeans would be a possibility.
When dividing proteins by total calories (see for instance USDA nutrient database, or here) soybeans are among the most protein-dense foods, and they are also relatively inexpensive, after all they feed much of the world's cattle.